From our Polar Cloud guide, infill is defined as: Each model to be printed is comprised of an exterior and an interior. While the exterior is typically printed solid with no holes or gaps, the interior may range anywhere from completely empty to completely solid. The plastic printed in the interior is known as “infill” and its solidity is the “infill percentage”. For example, 0% infill means the print is completely hollow and 100% infill means it is fully solid. Printing time is reduced and plastic is saved by printing with infill percentages significantly less than 100%. The amount of infill you should use depends upon the nature of the piece being printed and its intended usage. 

This is adjusted during your time on the Build Plate, visualizing the object.

For more info on Infill, check out Angus's video below!